However, the six-time Ballon d'Or winner is yet to taste senior success in an Argentina jersey following a runners-up finish at the 2014 World Cup, having also reached back-to-back Copa America finals in 2015 and 2016.
A two-time Argentina international, Pachuca goalkeeper Ustari warned against doubting captain Messi – the country's all-time leading goalscorer.
"I have no doubt that if Messi isn't world champion, football will be unfair," Ustari told Radio Mitre.
"Doubting Messi is nonsense. Things can go well or not, but I can tell you that I have seen him crying like a child because of what was happening in the national team. It's very difficult to be in his shoes.
"I remember that in the 2011 Copa America [Argentina were eliminated by Uruguay in the quarter-finals] I was recovering from my knee and, when they lost, I went to see Messi at the Ezeiza venue and I saw him like I had never seen him before, devastated."